The present invention relates to optical elements utilizing optical anisotropy to provide an optical low-pass filter effect and image-pickup apparatuses using the optical element such as digital cameras, video camcorders and the like with a solid-state image-pickup element (for example, a CCD sensor or a MOS sensor).
An image-pickup apparatus, such as digital camera and video camcorder, using a two-dimensional solid-state image-pickup element, such as a CCD sensor and a MOS sensor, acquire image information by sampling an object image on a pixel basis.
However, when an object having a spatial frequency component higher than the spatial frequency of the pixel pitch is picked up, the high frequency component is detected as an aliasing noise component at lower frequencies.
On the other hand, when a single-plate color image-pickup element is used to pick up an image of an object having a high spatial frequency component, there is generated a false-color noise determined by the layout of a color filter disposed in front of the pixels.
It is extremely difficult to remove such a noise component after the image information is converted into an electrical signal.
Conventionally, such a noise component has been removed using an optical low-pass filter.
The optical low-pass filter used herein is an optical member having an optical effect capable of removing a high spatial frequency optical signal. There is known an optical low-pass filter using a light separation effect of quartz (Japanese Utility Model Publication No. S47(1972)-18688, Japanese Utility Model Publication No. S47(1972)-18689, Japanese Patent Laid-Open No. S59(1984)-75222, and Japanese Patent Laid-Open No. S60(1985)-164719).
An optical low-pass filter using quartz utilizes a light separation effect by an anisotropic optical medium to obtain a low-pass effect.
In Japanese Utility Model Publication No. S47-18688 and Japanese Utility Model Publication No. S47-18689, a stripe filter is contemplated as a color filter and an optical low-pass filter is used to reduce a false-color signal generated when the spatial frequency of an object is synchronized with the color filter.
Specifically, the above two patent documents disclose an optical low-pass filter in which a plane-parallel plate with birefringence, such as quartz, is used to separate light into ordinary and extraordinary rays, which are then focused on an image-pickup plane.
Japanese Utility Model Publication No. S47-18689 discloses a configuration in which single-crystal quartz is cut out such that its optic axis is inclined substantially 45 degrees to the incident/emergent surface of the plane-parallel plate.
In Japanese Patent Laid-Open No. S59-75222 and Japanese Patent Laid-Open No. S60-164719, a Bayer-layout color filter is contemplated as a color filter, for example, and a plurality of birefringent plates are combined to separate entering light into ordinary and extraordinary rays, which then exit therefrom and form an image. This configuration effectively reduces a spurious-resolution signal and a false-color signal which are generated by high frequency components of an object.
There is further known an optical element having a light separation effect of an anisotropic medium that exhibits stronger anisotropy than that of quartz (Japanese Patent Laid-Open No. 2001-147404 and Japanese Patent Laid-Open No. 2002-107540).
Japanese Patent Laid-Open No. 2001-147404 discloses an optical low-pass filter with thinner mechanical thickness by using lithiumniobate, which exhibits stronger anisotropy than quartz, to provide a larger light ray separation width per unit thickness.
However, a material that exhibits too strong anisotropy typically provides too large width of separation, resulting in a too thin optical element for a desired width of separation. This leads to difficult material processing and reduced mechanical strength.
Therefore, in Japanese Patent Laid-Open No. 2001-147404, the angle θ between the normal to the surface and the optic axis of the optical element is set within a range of 10°<θ<30° or 60°<θ<80°, which deviates from 45° at which the width of separation is maximized.
In Japanese Patent Laid-Open No. 2002-107540, a single-crystal lithium niobate is cut into a parallel plate to form an optical low-pass filter.
Japanese Patent Laid-Open No. 2002-107540 discloses a configuration in which a normal to the principal plane of the single crystal that is cut out is within a range of ±3° around the z axis of the crystal from a position that is rotated around the x axis by 46.1±20° or 133.9±20° from the y axis.
The difference in the refractive indices of lithium niobate is typically greater than that of quartz, which facilitates achieving a thinner mechanical thickness.
When light enters a plane-parallel plate made of an anisotropic medium in which the angle between the optic axis and the normal to the incident surface thereof is specified, the width of separation of exit light rays varies depending on the incident angle of the light. That is, different optical low-pass filter effects will be provided for different incident angles of the light.
Thus, in an image-pickup apparatus using an optical low-pass filter, when light rays enter the optical low-pass filter at different incident angles, it is difficult to achieve a uniform optical low-pass filter effect on the entire image.
Therefore, in an optical low-pass filter using an anisotropic medium, it is important to appropriately set the refractive index of the material, the direction of the optic axis with respect to the normal to the incident surface, the thickness and the like to reduce variation in light separation width caused by variation in incident angle.
An object of the present invention is to provide anoptical element capable of reducing variation in light separation width with respect to variation in incident angle of entering light and having there by an excellent optical low-pass filter effect, and an image pick-up apparatus with the an optical element.
According to an aspect, the present invention provides an optical element having optical anisotropy, the optical element separating entering light into light rays having polarization directions perpendicular to each other such that the light rays are separated by a specific width and exit from the optical element. The optical element satisfies the following conditions:0.1<|ne−no|θmax<θwhere no and ne represent refractive indices for ordinary and extraordinary rays at a wavelength of 530 nm, θ represents an angle between the direction of an optic axis of the optical element and a normal to an incident surface of the optical element, and θmax represents an angle between the direction of the optic axis and the normal to the incident surface at which the angle of separation is maximized when the light that enters the incident surface of the optical element at the normal angle is separated into the light rays having the polarization directions perpendicular to each other.
According to another aspect, the present invention provides an optical element separating entering light into light rays having polarization directions perpendicular to each other such that the light rays are separated by a specific width and exit from the optical element. The optical element satisfies the following condition:50°<θ<60°where the optical element is made of lithium niobate, and θ represents an angle between the direction of an optic axis of the lithium niobate and a normal to an incident surface of the optical element.
According to another aspect, the present invention provides an optical element separating entering light into light rays having polarization directions perpendicular to each other such that the light rays are separated by a specific width and exit from the optical element. The optical element satisfies the following condition:51°<θ<61°where the optical element is made of quartz, and θ represents an angle between the direction of an optic axis of the quartz and a normal to an incident surface of the optical element.
According to yet another aspect, the present invention provides an image-pickup member which comprises the optical element according to one of the above ones, and a solid-state image-pickup element. The optical element is located on the light-entering side of the solid-state image-pickup element.
According to yet another aspect, the present invention provides an image-pickup apparatus which uses the above-described image-pickup member to photoelectrically convert an optical image formed by an image-pickup optical system.
According to yet another aspect, the present invention provides an image-pickup apparatus which comprises an optical element having optical anisotropy, and a solid-state image-pickup element which receives an optical image formed by an image-pickup optical system through the optical element. The image-pickup apparatus satisfies the following conditions:0.02<|ne−no|θmax<θp/t<0.035where no and ne represent refractive indices for ordinary and extraordinary rays at a wavelength of 530 nm, θ represents an angle between the optic axis direction of the optical element and the normal to the incident surface of the optical element, θmax represents the angle between the direction of an optic axis and a normal to an incident surface at which the angle of separation is maximized when light that enters the incident surface of the optical element at the normal angle is separated into light rays having polarization directions perpendicular to each other, t represents the thickness of the optical element in the direction of the normal to the incident surface, and p represents the pixel pitch of the solid-state image-pickup element in the direction of the light separation.
Other objects and features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to accompanying drawings.